Should one tank in tank stance?

[PyurBlue]

You should start off in tank stance. If there is another tank, it will signal to them that you want to main tank and it will also make establishing aggro easier no matter the content. In dungeons most tanks will keep tank stance on for mob pulls and then switch to DPS stance for bosses after aggro is generated. For raids, use tank stance to establish an aggro lead with a couple of aggro combos, then switch to DPS. The damage reduction loss isn't a big deal in these cases and for PLD in particular, you gain much faster Oath gauge generation which gives you the ability to consistently Sheltron and increase defense.

Staying in tank stance for the duration of the content isn't optimal, but it's not terrible. I don't mind it unless I'm the main tank and the offtank is the one that keeps tank stance on.

For dungeons, once sufficiently geared, you'd only need tank stance for large pulls. The individual bosses and other limited target fights such as primal fights, savage, etc. you'll only use your tank stance for initial enmity. The rest of the fight will be spent in your dps stance.
DRK and PLD can use their stance as an extra defensive cooldown if needed, though they rarely are.

You generally have two reasons people will be against this. Enmity and damage reduction.
Enmity is easy, everyone plays with it. This is why you pull in tank stance but swap out shortly after. Between diversion, lucid dreaming, and tactician/refresh, everyone has ways to reducing and lowering their enmity. Plus with Shirk, any tank swaps will only boost the MT's enmity further. It shouldn't be an issue.

I'll just add to this that enmity controls have moderately long cooldowns. Unless the party is willing to stop and wait between pulls, they might not always be available. I tend to not use enmity reduction on mobs outside of the first pull because I'd rather not rip the boss off the tank. Everyone has to watch aggro, but I think the tank bears a little more responsibility outside of boss fights.